Remembered fondly (and occasionally mockingly) as the episode where Captain Kirk built a homemade diamond cannon out of rocks and sticks, "Arena" almost perfectly encapsulates the Star Trek ethos. Kirk must outwit a stronger conventional foe — in this case, a rather ridiculously sluggish lizard-man — to save his ship from destruction. After he wins by cobbling together gunpowder from available minerals — proving brains over brawn — Kirk refuses to kill his enemy, thereby earning the moralizing approval of the godlike race that set up the allegorical contest in the first place. In the abstract, it's as cheesy as a one-ton quiche, but by some strange Roddenberry-esque alchemy it comes together into an iconic hour of television.

The apex of Star Trek's comedic streak, which introduces the most beloved threat to human civilization in Trek history: the tribble. Furry, featureless, and so cute it's painful, you'll overlook the tribbles' ability to violate the laws of thermodynamics as they exponentially consume and multiply like a mogwai tossed in a car wash. Fortunately, the tribble does have one legitimate practical application: detecting Klingons, whose mere presence makes the fertile furballs trill with terror. Yes, this is actually a critical plot point in "Trouble." Throw in a diplomatic conspiracy, Scotty getting involved in a smack talk throwdown with a Klingon crew, the obligatory inter-species bar fight, and more snark than you can shake a phaser at, and it all adds up to perhaps the most charmingly hilarious episode in the Trek annals.

Setting aside the fact that this episode introduces some of the most critical aspects of the Trek canon — the Romulans, the neutral zone, cloaking devices — this is an old-school character study and a meditation on the pointlessness of prejudice and war. Kirk squares off with a Romulan commander on a mission of stealth destruction, using a weapon that can wipe out a starbase in a single shot, only to find the two of them are kindred spirits. Spock, meanwhile, stares down bigotry and suspicion when it's revealed Romulans and Vulcans share a common ancestry. Pack that all in with the tension of two ships on silent running, where a single misstep means certain death. Even though our heroes prevail, it is a hollow victory, as they are forced to destroy a worthy opponent that, in a just universe, they could have called "friend."

The entire notion of evil twins sporting goatees started here, and that's maybe the 10th coolest thing about this episode. Kirk, Scotty, McCoy, and Uhura are transposed with their Mirror Universe counterparts (thereby introducing the Mirror Universe) during a transporter accident, introducing us to versions of Spock, Sulu, and Chekov that serve the conquest-hungry, Gestapo-like Terran Empire. It's an extremely potent storytelling device to have the characters act as foils for themselves, highlighting the heroism and moral vagaries of our familiar Trek protagonists by showing how they could use their talents and tech for selfish ends. Both Leonard Nimoy and Spock shine here. The former for portraying two versions of his classic character with such nuance; the latter for illustrating how a dispassionate, neutral stance can do as much damage as willful destruction, as all it takes for evil to triumph is for good Vulcans to do nothing. Assassinations, crazy costumes, an evil enforcer Sulu, a love story — and a moral, too? It doesn't get more Trek than that.

If all you knew is that Harlan Ellison wrote (and then later disavowed) this episode, that would be enough to give you pause. That Joan Collins plays Kirk's love interest ups the ante. That this is a rare excellent Trek time travel story — introducing the legendary Guardian of Forever - and also the original and ultimate test of Spock's utilitarian axiom "the needs of the many outweigh the needs of the few" should be enough to cement its status as the pinnacle of original Star Trek.

A disoriented McCoy is timewarped to 1930s America, where he saves social worker Edith Keeler (Collins) — who then goes on to found a peace movement that prevents America from joining World War II, and thus the Axis powers conquer the world. Kirk and Spock must follow McCoy to the past and ensure that Keeler dies — even after Kirk falls in love with the well-intentioned peacenik. It's the ultimate moral quandary, both of the dangers of time travel, and of weighing the life of one victim against the liberty of untold millions.

Kirk, once again, is caught between the compassionate aims of McCoy, who sees only an innocent life he has the power to save, and the cold logic of Spock, who sees the certain consequences of a single emotionally satisfying act. It's the one no-win scenario that Kirk failed to outwit, even if history — to say nothing of the television audience and the Trek franchise as a whole — ultimately won. Put simply, it's the best original series Star Trek episode ever, bar none.

Disagree with the rankings, or simply want to swap stories about the foremost five TOS episodes? Hailing frequencies are open in the comments section.

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About Jay Garmon

Jay Garmon has a vast and terrifying knowledge of all things obscure, obtuse, and irrelevant. One day, he hopes to write science fiction, but for now he'll settle for something stranger — amusing and abusing IT pros. Read his full profile. You can a...

Full Bio

Jay Garmon has a vast and terrifying knowledge of all things obscure, obtuse, and irrelevant. One day, he hopes to write science fiction, but for now he'll settle for something stranger — amusing and abusing IT pros. Read his full profile. You can also follow him on his personal blog.